Hirotake Kajii

Enhancement of electroluminescence utilizing confined energy transfer for red light emission

Organic light emitting diodes (OLED) have been attracting attention because of their advantages in emission in a wide visible region and for applications to flat-panels or as light sources for optical signal circuits or other applications. In this paper, we report the fabrication and emission characteristics of two kinds of electroluminescence devices for enhanced red light emission utilizing energy transfer and energy confinement. The device consists of two materials systems: one is a co-doped OLED which consists of two different kinds of dye materials doped in an emissive layer, and the other type is a europium (Eu) complex doped in poly(N-vinylcarbazole), which are discussed. The results show that, in both cases, the enhancement of emission can be observed by the energy confinement of carriers. The mechanism of enhancement of emission is discussed, in terms of carrier confinement and by utilizing an energy band diagram of the emissive materials.

Realization of polymeric optical integrated devices utilizing organic light-emitting diodes and photodetectors fabricated on a polymeric waveguide

Direct fabrication of organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs) on polymeric substrates, i.e., polymeric waveguide substrates to form flexile optical integrated devices is demonstrated. The OELD and OPD were fabricated by organic molecular beam deposition (OMBD) technique on a polymeric or a glass substrate, for comparison. The device fabricated on a polymeric substrate shows similar device characteristics to that on a glass substrate. Optical signals of faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing diamine derivative, or rubrene or porphine doped in 8-hydoxyquinolinum aluminum derivatives, as an emissive layer. Electrical signals are successively converted to optical signals for optical transmission of moving picture signals with OLED fabricated on a polymeric waveguide. OPDs utilizing phthalocyanines derivatives with superlattice structure provide increased pulse response with input optical signals, and the OPD with the cutoff frequency of more than 5 MHz has been realized.

Electrical and Optical Properties of Conducting Polymer - Fullerene and Conducting Polymer - Carbon Nanotube Composites

Abstract Various novel photo-physical properties such as a drastic photoluminescence quenching and photoconductivity enhancement and a photo-induced enhancement of low-field microwave absorption due to a highly effective photo-induced electron transfer have been observed in various conducting polymers doped with fullerenes and also acceptor-type molecules and conducting polymers. New types of junction devices utilizing effective charge separation at the interface of conducting polymer/C60, C60-doped conducting polymer/C60 and acceptor type conducting polymer / donor type conducting polymer have been proposed and their novel characteristics have been demonstrated. Novel concepts such as an interpenetrating network, a condensed interface and the effect of introduction of photo-harvesting antenna molecule at the interface have been discussed. Nano-composite films of carbon nanotube (NT) and conducting polymer were prepared. With increasing the volume fraction of NT in conducting polymer the conductivity incr...

Photoresponse Properties of a High-Speed Organic Photodetector Based on Copper–Phthalocyanine Under Red Light Illumination

For this letter, carrier generation and charge transfer characteristics in an organic photodetector under forward and reverse bias conditions were analyzed. For the forward bias condition, holes are dominantly injected into the highest occupied molecular orbital states of copper-phthalocyanine from an indium tin-oxide electrode and flow into the Au electrode. However, under a reverse bias condition, the photocurrent is generated at the hetero interface and removed by both electrodes. A cutoff frequency of 70 MHz was observed under a reverse bias of 7V and clear response pulses at 100 MHz were observed using a sinusoidally modulated laser (lambda=650 nm)

Red Phosphorescent Organic Light-Emitting Diodes Using Mixture System of Small-Molecule and Polymer Host

We study two types of polymer materials, poly(n-vinylcarbazole) (PVK) and starburst small-molecule 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB), as the host for electrophosphorescent organic light-emitting diodes (PHOLEDs) doped with red-emitting phosphor tris(1-phenylisoquinoline) iridium (III) [Ir(piq)3]. PHOLEDs employed a PVK and TDAPB blend as the host exhibited a maximum red light emission at a wavelength of 630 nm. The external quantum efficiency of 6.3% and power efficiency of 3.0 lm/W have been achieved. The electroluminescent (EL) spectra were not changed at high current and the luminance reached 8,800 cd/m2 at the voltage of 13 V. We found that the roughness of the surface as estimated by means of atomic force microscopy (AFM) smoothened with increasing TDAPB doping concentration, and a slight exciplex emission between TDAPB and an electron transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4,-oxadiazole (PBD) was observed at the wavelength of approximately 530 nm.

Transient Properties of Organic Electroluminescent Diode Using 8-Hydroxyquinoline Aluminum Doped with Rubrene as an Electro-Optical Conversion Device for Polymeric Integrated Devices

We describe modulation characteristics of organic light-emitting diodes (OLEDs) using 8-hydroxyquinoline aluminum (Alq3) as an electro-optical conversion device for polymeric integrated devices. Optical pulses of more than 100 MHz were created by directly modulating a yellow emitting OLED with rubrene doped in Alq3 as an emissive layer which has the advantages of low propagation loss in the polymer waveguide and high emission intensity. We demonstrate that the OLEDs can be applied to fields of optical communication as electro-optical conversion devices for transmitting the signals of moving images.

Organic light-emitting diode fabricated on a polymer substrate for optical links

An organic electroluminescence diode (OLED) fabricated on polyimide substrate has been investigated to realize a flexible electro-optic device for optical links. The device fabricated on polyimide substrate shows similar device characteristic to that on glass substrate. The modulation performance of the OLED was significantly improved by applying a positive, low-level DC bias, which results in a slower falling time. An undershoot voltage applied to the falling edge of the input voltage pulse results in faster falling time and a decrease in intensity of the optical output. We demonstrate that the OLEDs can be applied to short-range optical communication fields, such as electro-optical conversion devices for optical links.

Electrical and optical properties of conducting polymer-C60-carbon nanotube system

Nanocomposite of multi-wall carbon nanotubes (MWNT) and C 60 -doped conducting polymer have been studied. MWNTs were prepared by the method of organic chemical vapor deposition (CVD) using metal phthalocyanines as starting material. We have examined the electrical and optical properties of the nanocomposite film of MWNT - C 60 -poly(3-hexylthiophene) (P3HT) with different volume fractions of MWNTs, The enhancement of photoconductivity of composite films has been found at near percolation threshold. The enhanced photoresponse was observed due to the efficient charge separation and reduced recombination of photogenerated charge carriers in MWNT-C 60 -P3HT composite film.

Polymer Light-Emitting Diodes Fabricated Using Poly(9,9-dioctylfuorene) Gel by Thermal Printing Method

The optical characteristics of poly(9,9-dioctylfluorene) (F8) gel and polymer light-emitting diodes (PLEDs) fabricated using F8 gel by the thermal printing method were investigated. F8 gel was formed when the F8 content was more than a certain concentration in 1,2,4-trichlorobenzen with a high boiling temperature. F8 gel was transformed into F8 sol by annealing the F8 gel at approximately more than the glass transition temperature of F8. The transition between the F8 sol and gel was reversible. The occurrence of the β phase results in the formation of F8 gel. For this thermal printing method, the optical properties of F8 films and PLEDs can be controlled by the transfer speed of F8 gel, and the annealing substrate temperature. Blue, yellow-green, and red lights were emitted from the PLEDs using F8 gel, that doped with a yellow-green emissive fluorene copolymer, and that doped with a red emissive phosphor, respectively. We demonstrated the possibility of the polarized PLEDs using conducting polymer gels doped with emissive dopants by the thermal printing method.

Fast Response of Organic Photodetectors Utilizing Multilayered Metal-Phthalocyanine Thin Films

Multilayered photodetectors have been fabricated by using metal phthalocyanines as hole and electron transporting layers. From the dark and photocurrent density and applied voltage characteristics, the open-circuit voltage and short-circuit current density for the 5 periods of multilayered photodetector are about 0.3 V and 0.2 mA cm-2, respectively. The photoresponse, irradiating pulsed light with the wavelength of 640 nm, has also been measured. The photogenerated excitons dissociate to free holes and electrons by relatively rapid charge transfer across closely stacked organic layer interfaces. The 3 dB bandwidth of the device has been improved by applying a reverse bias field to the device and by decreasing the individual photo-absorbing layers while keeping the total layer thickness constant. A cutoff frequency of 1 MHz has been obtained for 5 periods of multilayered titanyl phthalocyanine/fluorinated zinc phthalocyanine photodetector by applying a reverse bias voltage of -5 V.